Our major effort over the next year will still be to determine the relationship of the initial Pi burst to the transition from the refractory to the non-refractory state. At the present time using subfragment-one (S-1) at 15 degrees in the absence of KCl, we find that the rate of the initial Pi burst is 2 to 3 times the rate of the transition from the refractory to the non-refractory state. This is not a large enough difference to be certain that they are separate steps. Therefore we plan to repeat these experiments using chymotryptic digestion to prepare S-1 which can then be separated into two fractions each of which contains a different alkali light chain and shows a different actin-activated ATPase rate. By comparing the initial Pi burst and the actin activation at varying pH and ionic strength with these preparations we should be able to determine the relationship between the initial Pi burst and the transition from the refractory to the non-refractory state quite accurately. In our theoretical work with Terrell Hill, we plan to quantify our cross-bridge model to determine whether it will fit current physiologic data without making any further assumptions.